A wireless local area network (“WLAN”) such as an IEEE 802.11-based network enables wireless devices within the coverage area of the network to communicate with one another and, typically, with other devices that are coupled to another network. For example, an 802.11-based access point may include a radio for communicating with mobile stations within its coverage area and also some form of connection to another network (e.g., a wide area network such as the Internet).
In some applications several WLANs may be deployed as neighboring networks to provide a wider collective coverage area. For example, in an enterprise deployment several access points may be located throughout the facility (e.g., a building or campus) such that the coverage areas of adjacent access points overlap to some extent. In this way, a mobile station may maintain local area network connectivity as it roams through the enterprise facility. That is, as the wireless station moves from the coverage area of one WLAN to the coverage area of another WLAN the station may disassociate from the first WLAN and associate with the second WLAN.
Here, provisions may be made to provide a station with information regarding WLANs in the immediate vicinity to enable the station to efficiently roam between neighboring WLANs. For example, a station may continually monitor the signal strength of signals received from nearby WLANs. In this way, the station may determine which WLAN provides the best connectivity at a given geographical location. A station may thereby elect to switch from one WLAN to another in an attempt to maintain a high quality of service.
The 802.11 standard defines both active and passive techniques for a station to acquire information regarding neighboring WLANs. For example, a station may actively scan the 802.11 channels by successively sending a probe request over each channel and waiting for a probe response from any nearby access points operating on that channel. The station may thereby obtain information regarding neighboring WLANs via information provided in the probe responses. A station utilizing active scanning, however, expends power and increases the load on the wireless channel by repeatedly transmitting probes. In addition, in some areas (e.g., different countries) a station may not be authorized to transmit probes on certain channels. Hence, additional provisions may need to be taken to avoid potential regulatory issues associated with active scanning.
Alternatively, a station may passively scan for traffic on each of the 802.11 channels to determine whether there are any nearby WLANs. To determine more information about any access points deployed in a given channel, the station may scan for beacons transmitted by each access point. In some applications, however, an access point may have a relatively long beacon interval (e.g., on the order of a second). Hence, a station may expend a considerable amount of power scanning for beacons for relatively long periods of time on each channel. Moreover, in the event the station is associated with an access point on a given channel, data transmissions between the station and the access point may be adversely affected when the station is scanning on other channels.
To improve the efficiency of active or passive scanning operations, 802.11 specifies a technique for providing a list of neighboring access points to any stations in the immediate area. For example, an access point may send a neighbor report to each of its associated stations. The neighbor report may include the list of neighboring access points, the channel employed by each access point, the beacon interval for each access point, the time synchronization function (“TSF”) offset for each access point, and other information.
Through the use of such information, a station may more efficiently scan for beacons from each of the neighboring access points. Specifically, a station may determine when and on which channels the beacons will be sent. Consequently, the station may not need to passively scan for long periods of time or employ active scanning to acquire information (e.g., associated signal strength) relating to neighboring access points.
In practice, the target beacon transmit times (“TBTTs”) of the various access points in a basic service set affect how quickly a station may acquire information about the neighboring access points on the various channels. For example, if beacons are sent at substantially the same time on two or more different channels, the station may need to wait several more TBTTs before it can decode all of the beacons of the neighboring access points.